Monolayer Ge2P4S2 and Ge2As4S2 are proposed as a new type of efficient photocatalyst for water splitting, based on first-principles calculations. Monolayer Ge2As4S2 exhibits a direct band gap of 1.89 eV (based on HSE06 calculation), while monolayer Ge2P4S2 is an indirect gap semiconductor that can turn into direct band gap by applying 3% compressive strain. Moreover, the band edge positions of monolayer Ge2P4S2 and Ge2As4S2 perfectly cover the redox potentials of water. Remarkably, the Ge2P4S2 and Ge2As4S2 monolayers possess rather high carrier mobilities (similar to 10(3)-10(4) cm(2) V-1 s(-1)) have moderate optical absorption performance in the range of visible light. In addition, the adsorption and decomposition of water molecules on monolayer Ge2P4S2 and Ge2As4S2 are explored to illustrate the mechanism of photocatalytic hydrogen formation. These results demonstrate that the mono layer Ge2P4S2 and Ge2As4S2 hold great potential for photocatalytic water splitting. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.